1. Technical Field
The present invention relates generally to document sorting and processing, and more particularly relates to an adaptive and predictive document tracking system for high-speed document processing.
2. Related Art
The field of high-speed document processing requires the use of machines and systems capable of moving and processing very large volumes of documents at rates of thousands of documents per minute, while performing multiple and interrelated operations upon each document as it travels through such machinery. Such operations might include, but are not limited to, printing, reading encoded data, recording archival images, etc. One such exemplary system is a check sorting device, commonly used by banks and other financial institutions (e.g., the IBM 3980™ Check Sorter, i.e., “3890”).
In a document processor such as the 3890, there are many document sensors located throughout the machine to detect document presence or absence at each location, while the documents are traveling at high speeds through a transport. If a jam develops or documents are mislocated at a particular location, intervention under machine control is necessary to ensure minimal or no damage to customer checks. If machine performance is poor in the tracking function, expensive manual remedies result to clear jams, with the possibility that damage to customer checks is so severe that the information on the check is unrecoverable and lost. This is a very undesirable result, and any improvement in the tracking function to make the system performance more robust and reliable has much value.
A key factor in the performance of document tracking is the behavior and reliability of the document sensor, which is a pneumatic sensor 40 in the 3890 as shown in FIG. 1. The sensor 40 operates with opposing airflow streams 48 (in/out of the page), which in the absence of a document causes an elastic diaphragm 50 to deflect. The diaphragm 50 has on its surface a spiral conductive pattern, which, with the deflection mentioned, makes contact with a conductive plate 52, making a closed contact (a logic zero) for detection of no document. When a document passes between the sensor 40 and one of the air flow streams 48, the pressure within the sensor 40 is reduced to the point where deflection of the diaphragm 50 is insufficient to cause contact between the two conductive surfaces, yielding a logic one indicating the presence of a document. The sensor 40 has ground 54 on one of the conductive surfaces 52 and a pull up resistor 46 to logic supply voltage 44 on the other conductive surface 50, and this latter node then swings between ground 54 and the logic supply voltage 44 as an input signal 42 to subsequent filtering and logic circuitry.
As shown, the pneumatic sensor 40 includes a grounded plate 52 and an arc 50 representing the elastic diaphragm, which deflects depending on pressure of the net air stream through the device represented by the black circle 48. The pneumatic sensor 40 has behavior modes and imperfections that can cause errors in the document tracking function. Some of those are contact bounce, glitching, stuck contacts (open or closed), unusually high resistance when closed, etc.
The filtering and processing of pneumatic sensor data up to now has been done in two stages, the first stage being a very simple first order low pass filter with a time constant of approximately 270 microseconds. This simple filter output is digitized with a Schmitt trigger stage of a comparator with positive feedback, yielding hysterisis thresholds of ⅓ and ⅔ the logic supply voltage. The pneumatic sensor actuate/deactuate delay has been in the few hundred microsecond region. Glitching and contact bounce can occur with pulse widths from a few microseconds to a few hundred microseconds, the larger values being sufficient to get through the filter and cause logic errors, false indications of document presence or absence, etc.
One of the fundamental problems that exist with present day check sorters is the sometimes poor performance of the sensors used for document tracking. Because of the high speeds at which these machines operate, they are often subject to erratic sensor readings. Erratic sensor output leads to loss of accurate document location information in a high-speed transport where documents are running close together. The resulting failures may include document jams, documents that are incorrectly sorted, auto-selects (rejects) due to inadequate processing time for various features located throughout the transport, etc. The net result then involves expensive manual corrective procedures to address the failure.
In order for a sensor to operate properly, it must accurately generate a signal when an edge of a document is detected. However, sensor signals contain glitches, undesired false pulse outputs of significant pulse width, excessive delay of document edge information, and other undesired behavior. Accordingly, failures are primarily caused by lack of clean sensor signal transitions to indicate document edges. To address this, today's sensor handling technology utilizes processing techniques, including a first order filter to clean up the signal. Unfortunately, existing filtering and processing techniques fail to yield acceptable performance for the range of sensor behaviors that are experienced. For the purposes of this disclosure, glitching is defined broadly as any ambiguous signal, e.g., a signal having a strength (voltage) and duration (pulse widths) atypical of a sensor reading.
Very significant engineering effort has been spent on improving the materials and process for manufacture of the sensor itself. In spite of this, the failure rate of new sensors, and the replacement rate of the field install base of sensors leads the sensor to be one of the most expensive items in the field service budget. However, not all sensors that are thought to be defective in the field are found to be defective in later testing.
Accordingly, a need exists for more robust sensor handling systems that can address the erratic behaviors found in many of today's high-speed document machinery.